That solely depends on the question what you consider “Casual”. I’ll go with the definition “a game that doesn’t need a significant time investment per play-session”.

• Super Hexagon: Amazing music, simple game, but challenging.
• Triptych: Tetris with soft body physics. The game is great, the sound a bit annoying. Also, I don’t think it’s available on Steam, so it needs manual installation (and input bindings).
• Cultist Simulator: While the game is very obscure, it doesn’t require a lot of time per session - you can start it, play 5 minutes, and exit it again.
• Hades: It’s a Roguelite. You die. A lot. That’s why each session can be rather short.
• Loop Hero: It’s not as “casual” as the others, as it takes some time for each run, but not too much.